Abstract
A 36-year-old female noticed pigmented patches on her back, neck, and buttock after a few days of oral administration of S-carboxymethyl-L-cysteine (SCMC) and dimemorfan phosphate. We presumed fixed drug eruption (FDE) induced by SCMC and therefore performed a lymphocyte stimulation test for SCMC. However, the result was negative. The result of patch test with SCMC was also negative, but that with its night-time intermediate metabolite thiodiglycolic acid (TDA) was positive. We definitely diagnosed the patient with SCMC-induced FDE because her skin eruptions were induced on the third day after consecutive ingestions of SCMC during the oral challenge test. It has been suggested that TDA could possibly be the causative allergen of SCMC-induced FDE. Several studies have reported that performing a patch test with TDA was useful for the diagnosis of SCMC-induced FDE. In our case, the result of the patch test with TDA was also positive. However,patch testing with TDA showed similar positive responses on healthy volunteers, so we carried out further research. To evaluate the usefulness of patch test with TDA in the diagnosis of SCMC-induced FDE, we performed patch tests with different concentrations of TDA on our patient and five healthy volunteers, and then assessed the response. The reaction occurred in a concentration-dependent fashion. We therefore measured the acidity of an aqueous solution of TDA and found that it was a strong acid. Infiltrative erythema on the patch test with TDA is possibly an irritation response, so physicians should be cautious when interpreting the results of patch testing with TDA in the diagnosis of SCMC-induced FDE.
